How do I get rid of the smell of decomp in my car?

A mouse (we're pretty sure) got into the vent opening behind the battery and apparently died in there. None of us want to use that car - it's smelled horrible for about a month now. Even with the windows and sunroof open. It's worse when the air is running through the vent - the a/c, heater, or just outside air. My husband tried to get to it from both the inside of the car and under the front hood but is unable to access the area. Does anyone have any ideas or has anyone dealt with a thing like this before? I'm afraid it will be extremely expensive if we take it to the dealership to have the vents pulled out and cleaned, if they even do that, it would take lots of labor hours. The car is a nice car, a 2004 infiniti M35X - if that would help anyone out there with any knowledge of this type of vehicle. Thanks for any help you can give us.